This invention relates to calcined powder of coagulated spherical particles having a secondary coagulated particle size of 0.2 .mu.m to 3 .mu.m, composed of primary particles of less than 500 .ANG. in the crystallite size, and obtained by calcining ultrafine zirconia particles with or without other metal compounds, as well as a process for producing them.
As the performances of fine ceramics have been improved in recent years, the size of the particles used for the starting material has tended to be reduced extremely and the importance of the ultrafine starting particles has gradually been increased also for zirconia ceramics, particularly, partially stabilized zirconia ceramics noted as toughed ceramics. Then, the use of fine particles of submicron size or ultrafine particles of less than about 500 .ANG. has now been started.
However, those fine particles, particularly, ultrafine particles of less than about 500 .ANG. involve serious troubles in view of the production of ceramics at least with respect to the following two points. One of them concerns the production of powder and the other of them arises in obtaining green compacts or forming products from the powder.
Generally, ultrafine particles of less than 500 .ANG. result in firm inter-particle bondings at the drying and calcinating step for the preparation of powder due to the effect of water or the like thereby tending to form irregular lumps, which can not be powderized with ease into a uniform fine particle size again even after pulverization and thus the properties of ultrafine particles are almost lost. Then, it is usually necessary to prevent firm and irregular coagulation by replacing the water content with an expensive alcohol or the like prior to the drying or to employ freeze-drying which, however, complicates the production procedures and results in economical disadvantage (refer to Japanese patent publication No. 25523/1979).
The other drawback is that the powdery ceramic material having an ultrafine particle size gives an extreme difficulty for molding even if firm and irregular coagulation can be prevented, in which the sintering shrinkage is increased, deformations or crackings are liable to be caused and the dimensional accuracy after the sintering is worsened.
The granulating method by the spray drying may be employed as a technical means for improving the uniform and dense packing property in the forming. However, since the use of water degrades the properties of the ultrafine particles as described above, the granulating is carried out after coagulating them through calcination. The coagulated particles obtained in this case lose their properties as the ultrafine particles and become inhomogenous which, after all, renders the inner texture of the green compacts or forming products inhomogenous.